I have had this plant in one of my gardens for two years, and all it has ever produced is leaves. I wasn't sure if it was a variety of iris or lily. This year it has finally bloomed, and the blossoms look like a lily. But I am still clueless as to the variety. The flowers are fairly small, maybe 2" across, and they seem to either close at night or only bloom for one day. If anyone knows what this is, please leave me a comment. Thanks!
UPDATE: The overwhelming majority of responders have identified this as a blackberry lily, which is actually a member of the iris family. I will know for sure once the seed pods form and break open to reveal the "blackberry" --- actually a cluster of shiny black seeds.